There are more than 200 crude oils produced and traded worldwide. Crude oils are very complex mixtures of many thousands of different hydrocarbons. Depending on the source, the oils contain various proportions of straight and branched-chain paraffins, cycloparaffins, and naphthenic, aromatic and polynuclear aromatic hydrocarbons. The nature of the crude oil governs, to a certain extent, the nature of the products that can be manufactured from it and their suitability for specific applications.
Worldwide supply and demand, regional refining capacities and configurations, and crude composition are the key factors that determine the value of crude oil. The first factor is purely market-dependent and cannot be predicted from the crude oil quality. Accordingly, the crude oil value is determined by the regional crude market and differentials such as freight, quality adjustments, refining cost and competitive pricing.
In a typical petroleum refinery, crude oil is first distilled under atmospheric pressure. Gases will rise to the top of the distillation column, followed by lower boiling liquids, including, naphtha, kerosene and diesel oil. Naphtha is not a final product, but is subjected to additional treatment steps, such as hydrotreating and catalytic reforming to produce reformate. The reformate is then sent to a gasoline pool for blending.
An article by Colin Birch, “Achieving Maximum Crude Oil Values Depends on Accurate Evaluation,” Oil & Gas Journal, Vol. 100, Issue 2 (Jan. 14, 2002), describes a number of evaluation methods for obtaining an objective calculation of the value of a specific crude oil from a particular source. Summaries of several of these methods follow.
Bulk-Property Method: This method correlates actual crude value with bulk properties. API gravity and sulfur content are widely used for the correlation, and other bulk properties, such as viscosity and pour point, can also be used. This method is relatively simple in terms of the amount of testing required. However, this method may not be reliable when a large range of crudes are being valued. For example, some of the naphthenic crudes may be valued relatively higher, using this method, but this result may not reflect the actual market value for the crude oil.
Refining-Value Method: Crude oils are evaluated and valued using the refinery yields and process operating costs for each crude stream, typically using a linear program (LP) or other model. Refinery models require detailed physical property information and distillation cuts as determined by a detailed crude oil assay. Process yields and operating costs are used with appropriate product values to calculate refining-value differentials between the crude oils. The refining-value method simulates the process used by refiners for selecting crude oils. Detailed crude oil quality information and the need to run a refinery model for a given refinery to generate the yields make this method more complex than the bulk-property method. If input stream quality changes significantly, a new set of yields must be generated. In relatively simple systems involving only a few crudes with reasonably stable quality, the refining-value method normally provides the most accurate value allocation for a refiner.
Distillation-Yield Method: This is a simplified version of the refining-value method, which instead of using a linear program or other model will only use the yield of each fraction. These product yields from distilling each crude are used with product values to calculate the relative value of each crude. In many cases, some physical properties of the distillation cuts are used in the value-adjustment system. The quality information from each crude is relatively simple and includes distillation yields and distillation cut properties. The distillation yield-method is more complex than the bulk-property method, but less complex than the refining-value method. Because it uses product values in the calculation, reliability of crude oil value data is not an issue. The products being valued, however, such as naphtha, are not finished products meeting defined specifications. So there is some uncertainty regarding the value adjustment for key properties of the distillation cuts.
Several properties of naphtha streams can be evaluated, including API gravity, sulfur, nitrogen, carbon and hydrogen contents, and research octane number. Research octane number is the measure of a fuel's ability to prevent detonation in a spark-ignition engine. Measured in a standard single-cylinder, variable-compression-ratio engine by comparison with primary reference fuels, American Standard Testing Material Tests ASTM D-2699 and ASTM D-2700 describe the determination of research and motor octane numbers, respectively. Under mild conditions, the engine measures research octane number (RON), while under sever conditions the engine measures motor octane number (MON). Where the law requires posting of octane numbers on dispensing pumps, the antiknock index (AKI) is used. This is the arithmetic average of RON and MON, namely, (R+M)/2. It approximates the road octane number, which is a measure of how an “average” car responds to fuel. It is the most critical property for naphtha/gasoline streams.
The RON of a spark-ignition engine fuel is determined using a standard test engine and operating conditions to compare its knock characteristic, defined as knock intensity (KI) with those of primary reference fuel (PRF) blends (containing iso-octane and normal heptane) of known octane number. For example, an 87-octane gasoline has the same octane rating as a mixture of 87% iso-octane and 13% n-heptane. Compression ratio (CR) and fuel-air ratio are adjusted to produce standard KI for the sample fuel, as measured by a specific electronic detonation meter instrument system. A standard KI guide table relates engine CR to octane number level for this specific method. The fuel-air ratio for the sample fuel and each of the primary reference fuel blends is adjusted to maximize KI for each fuel. While gasoline will have an RON of 85 or higher, naphtha will have an RON below 60.
The MON of a spark-ignition engine fuel is determined using a standard test engine and operating conditions to compare its knock characteristic with those of PRF blends of known octane number. CR and fuel-air ratios are adjusted to produce standard KI for the sample fuel, as measured by a specific electronic detonation meter instrument system. A standard KI guide table relates engine CR to octane number level for this specific method. The fuel-air ratio for the sample fuel and each of the PRF blends is adjusted to maximize KI for each fuel.
To determine these properties of gas oil or naphtha fractions conventionally, these fractions have to be distilled from the crude oil and then measured/identified using various analytical methods that are laborious, costly and time-consuming.
Therefore, a need exists for an improved system and method for determining the value of crude oils from different sources that can be objectively applied to compare the naphtha fractions from different sources.